MADRAS, India  India will launch a census of its vultures, a group of ornithologists said on Sunday, as the birds are vanishing rapidly due to a mystery virus and shrinking nesting sites.

The population of vultures, nature's scavenger and rodent controller, has fallen from tens of thousands just a decade ago to a few thousands also because of a veterinary drug found in cattle carcasses that the birds feed on, experts say.

"The vulture population in India has declined over 80 percent over the last few years. We have decided to conduct a census in select sanctuaries and reserve forests in view of that," said V.S. Vijayan, director of the Madras-based Salim Ali Centre for Ornithology and Natural History.

The census, which will be launched from northwestern state of Rajasthan later this month and cover 20 national sanctuaries, will also study the feeding and flight patterns of the species, he said.

The dramatic drop in the population of the bird has created a crisis for the country's Parsi community, which leaves its dead on top of stone towers to be eaten by vultures as its religion forbids burial and cremation.

Parsis, or Zoroastrians, regard fire, earth and water as sacred and believe the vulture helps release the spirits of their ancestors.

K. Venkataraman of the National Biodiversity Authority said the vultures also were threatened by increasing air traffic over Indian cities.

"Increase in air traffic has caused more number of vulture deaths in recent years due to airplane hits," he said.

Alarms over reports of a sharp drop in population of tigers and wild elephants have prompted Indian authorities to order their censuses too.